1. Field of the Invention
The present invention relates generally to a broadband wireless communication system. More particularly, the present invention relates to an apparatus and method for improving the reception performance of a Mobile Station (MS) by reducing Other-Cell Interference (OCI) in a broadband wireless communication system.
2. Description of the Related Art
In a broadband wireless communication system, when an MS is located at a cell boundary, far from its serving Base Station (BS), it suffers from a decreased reception performance due to path loss. When the distance to a neighbor BS is approximately the same as the distance to the serving BS, the MS may receive signals with similar power levels from the serving BS and the neighbor BS. Therefore, the signal from the neighbor BS interferes with the signal from the serving BS.
In a broadband wireless communication system a handover process is used to connect an MS from a serving BS to a neighbor BS that offers a better link. The handover process ensures that the MS continues to receive an on-going service without interruption.
The handover ensures the mobility of the MS and enables the MS to select a better link. The broadband wireless communication system makes a decision as to whether handover is required for the MS by assessing the link statuses between the MS and BSs based on the Signal-to-Interference and Noise Ratios (SINRs) of signals received from the BSs at the MS. That is, the decision is made using the average SINRs of channels associated with path loss and shadowing (i.e. the long-term statistics of the channels).
Another approach to improving the reception performance is Fast Cell site Selection (FCS). In the FCS scheme, the MS at the cell boundary selects a cell (or sector) offering a better link according to instantaneous SINRs reflecting fast fading of channels.
As mentioned earlier, if an MS is located at a cell boundary, it is at similar distances from a serving BS and a neighbor BS. The MS receives signals with similar average reception power levels from the serving BS and the neighbor BS, and experiences independent fast fading from the BSs. The MS can achieve a site selection diversity gain by receiving data selectively from a BS that offers the better link status at a given time instant by the FCS scheme.
FIG. 1 illustrates a configuration of a conventional broadband wireless communication system for performing FCS.
Referring to FIG. 1, a broadband wireless communication system includes a router 100, a first BS 110 (BS1), a second BS 120 (BS2) and an MS 130. The router 100 provides data to both the first and second BSs 110 and 120. The MS 130, located at a cell boundary, determines an active set including first and second BSs 110 and 120 (BS1 and BS2), for example, based on the link statuses of their cells (e.g. average SINRs) in order to perform FCS.
The MS 130 then measures the instantaneous SINRs of signals received from the active cells of the active set and selects the cell that offers the best link to camp on. For instance, the MS 130 measures the instantaneous SINRs of BS1 and BS2 and selects one of the BSs from which the MS 130 will receive a downlink signal according to the instantaneous SINRs. Then the MS 130 receives data from the selected BS.
As described above, the MS selects the link having the best channel status by measuring the instantaneous SINRs of neighbor cells in the broadband wireless communication system. As the MS receives data from the selected cell, it can achieve a site selection diversity gain.
If the MS is located at a cell boundary, the instantaneous SINRs rapidly drop because of OCI from neighbor cells, as illustrated in FIG. 2.
FIG. 2 is a graph illustrating variations of instantaneous SINRs according to locations of an MS in a cell area in the conventional broadband wireless communication system. In the illustrated case of FIG. 2, the instantaneous SINRs are compared between the case where the MS is at the center of a cell (e.g. d=300 m) and the case where the MS is at a cell boundary (e.g. d=2300 m). The instantaneous SINRs are normalized to average SINRs, for comparison.
As the graph reveals, when the MS is at the center of the cell, it is affected less by OCI due to path loss. Therefore, the MS does not suffer from as great a change in instantaneous SINR.
On the other hand, when the MS is at the cell boundary, its distance from a serving cell and neighbor cells is similar and thus it receives signals with similar power levels from the serving and neighbor cells. As a result, the MS suffers from fluctuating changes in instantaneous SINR due to OCI.
The fluctuation of instantaneous SINR places the MS in different channel statuses when the MS monitors its channel status and when it transmits actual data. This means that the MS may not select a cell offering a better link because of the instantaneous SINR changes caused by OCI.